Required and optional fields example

Main.purs

module Main where

import Prelude

import Effect (Effect)
import Data.Foldable (fold)
import TryPureScript (h1, h2, p, text, list, indent, link, render, code)
import Prim.Row (class Union, class Nub)
import Type.Row (type (+))
import Record as Record

type OptionalFields r = ( x :: Int, y :: Int | r )
type RequiredFields = (a :: Int)
type AllFields = OptionalFields + RequiredFields

goo ::
  forall given allFields.
  -- We need a separate allFields type variable because it's not the same as the AllFields synonym,
  -- in that it will have duplicate labels. That's what the Nub constraint is for.
  Union given (OptionalFields ()) allFields =>
  Nub allFields AllFields =>
  Record given ->
  String
goo r =
  let
    defaults :: Record (OptionalFields ())
    defaults = { x: 2, y: 3 }
    
    m :: Record AllFields
    m = Record.merge r defaults
  in
    show m

main :: Effect Unit
main =
    render $ fold
      [ h1 (text "Try PureScript!")
      , p (text (goo { a: 3 }))
      
      , p (text (goo { a: 1, x: 10 }))
      , p (text (goo { a: 1, y: 10 }))
      , p (text (goo { a: 1, x: 9, y: 10 }))
      
      -- doesn't compile
      -- , p (text (goo { x: 2 }))
      ]